The present invention is directed to an improved electrochemical method for continuously generating chlorine and/or bromine and to the apparatus required to perform the method. Specifically, the method is directed to the electrolysis of hydrohalic acid of an aqueous hydrohalic acid solution, continuously removing hydrogen and halogen gaseous products with simultaneous removal of portions of the electrolyte water from the electrochemical cell while maintaining a substantially invariant electrolyte solution.
The present process and device provides a simple, highly efficient means of producing halogen product in high yields without the need for an elaborate divided cell while preventing explosive concentrations of hydrogen to develop within the system and providing for substantially reduced corrosion problems.
The use of halogens, either individually or as mixtures, for water treatment is well known. Chlorine, bromine and their mixtures are known effective biocides for cooling water and swimming pool facilities, are useful for sterilization of drinking water and for the disinfection of waste materials. However, the use of halogens (The term "halogen" or halogens" shall refer herein and in the appended claims to chlorine and bromine unless otherwise specifically designated.) has been curtailed, especially at small to moderate size facilities, because of the difficulty and hazards involved in their transportation, storage and handling. For example, it is well known that storage and handling of halogen gases requires extreme care and precautionary measures. In the event of an accidental leakage of a large quantity of the halogen gas, considerable harm could come to the surrounding environment and to individuals in the vicinity. Because of the problems associated with handling and storage of pure halogen gases, there has been a great effort to develop either alternate means of providing halogen based biocides, as well as on-site generation of halogen gas as it is needed.
Concentrated solutions of sodium hypochlorite has been used in place of chlorine gas. Precautions must be maintained in handling hypochlorite solutions as exposure to them is known to cause severe damage to humans. In addition, such solutions have only limited shelf life and, therefore, must be used within a short period of a few weeks. Finally, there is a significant increase in cost and, thus, an economic barrier to use the hypochlorite solution in place of pure chlorine gas, especially in larger scale applications. Similar hypobromite solutions are not commercially available as these solutions are extremely unstable having the ability to remain active for only several hours to a few days.
Another alternative is the utilization of "halogen carriers." These carriers are usually solid organic compounds capable of releasing halogen when dissolved in water. For example, bromochlorodimethyl hydantoin (BCDMH) is a compound known to dissociate in water to form hypobromous and hypochlorous acids. Thus, compounds, such as BCDMH, provide a safe, convenient and easy to use form of providing halogen. The major drawback is the high cost of these materials which limits their practical use to only very small applications.
Recently, electrochemical halogen generators have received a great deal of attention as a means of generating halogen at the site of use and, thus avoid the transportation, handling and storage problems. The raw materials, alkali metal salts and electricity are safe and easily handled and the cost of halogen generation by this method is typically much lower then that of halogen carriers. Because this means is normally conducted in aqueous systems, hydrolysis occurs causing the ultimate product to be hypobromous or hypochlorous acid and, therefore, these generators are sometimes referred to as hypobromite or hypochlorite generators. Typical generators are disclosed in U.S. Pat. Nos. 3,305,472 and 4,208,123. In addition, J. P. Willington et al. in "Modern Chlor-Alkali Technology," Vol. 3, Pg. 400-404 (1986) Ed. by K. Wall, describes several electrochemical devices for generating hypochlorite in a continuous manner. Each of the devices require the use of precious metal anodes to inhibit corrosion and produce a dilute hypochlorite stream (0.5 to 2 g/l) from a 3 weight percent sodium chloride feed stream. These systems are, therefore, only applicable when appreciable amounts of salt can accompany the hypochlorite product.
In general, electrochemical halogen generators use dilute brine as the feed and, therefore, are susceptible to electrode fouling and corrosion due to precipitation of calcium and magnesium compounds at the cathode. The generators have design features which limit them to production of bromine or chlorine but are not interchangeably or capable of providing mixtures of halogens. The generators are normally required to be used in a batch mode either because of depletion of feed material and/or because of maintenance and antifouling work must be conducted on the system. Such requirements cause the known generators to be inefficient and cumbersome to handle. A further defect of presently known generators is the requirement that a diaphragm be used to separate the cathodic and anodic portions of the cell and the product streams generated by each portion. These diaphragms reduce the effectiveness and efficiency of the cell and are another element which undergoes corrosion from the generated elements. Overall, the known electrochemical generators provide low conversion, are of low efficiency, require periodic supervision to cleanse the system of spent material and replenish feed stock and require elaborate and expensive parts (i.e. electrodes) and equipment to overcome the corrosive nature normally encountered.
It is highly desired to provide a means for generating halogens by an electrochemical process which permits continuous operation of the system, permits the use of conventional low cost equipment and electrodes, provides for high efficiency and yield of halogen, does not need an elaborate divided cell configuration and permits the production of either chlorine or bromine or mixtures thereof utilizing the same equipment and method.